1. Field of the Invention
The present invention relates to a method and apparatus for reducing the amount of external fluid which travels through an opening into an enclosed area without impeding movement of a solid object through the opening and without impeding optical access through the opening. The present invention also relates to a method and apparatus for protecting a surface or area plane from contact with or intermixing with an external fluid.
2. Background Art
Use of gas jets and air curtains to direct the flow of gas into a desired space or to control the composition of gas in an enclosed area has enabled the improvement of numerous processes in recent years. Examples of such processes include: Maintaining an inert atmosphere in a chamber in which a cross-linkable polymeric coating is radiation cured; maintaining a non-oxidizing atmosphere above an ultrasonic solder bath; and, reducing heat loss from an industrial oven in which articles are heat treated. Some of the most useful applications of gas jets or curtains are in the prevention of external fluids from entering a process space wherein a continuous conveyor must move parts through the process space. U.S. Patents describing the use of gas jets or curtains in this latter manner include: U.S. Pat. No. 3,676,673, to Coleman, entitled: APPARATUS FOR IRRADIATION IN A CONTROLLED ATMOSPHERE; U.S. Pat. No. 3,807,052 to Troue, entitled: APPARATUS FOR IRRADIATION OF A MOVING PRODUCT IN AN INERT ATMOSPHERE; U.S. Pat. No. 3,936,950, to Troue, entitled: METHOD OF INERTING THE ATMOSPHERE ABOVE A MOVING PRODUCT; U.S. Pat. No. 4,298,341 to Nowacki, entitled: INDUSTRIAL OVEN HAVING MEANS FOR MINIMIZING HEAT LOSS; U.S. Pat. No. 4,448,616, to Francis, Jr. et al., entitled: PROCESS FOR REDUCING BACKMIXING; and U.S. Pat. No. 4,696,226 to Witmer, entitled: FLUID BARRIER CURTAIN SYSTEM.
U.S. Pat. No. 3,807,052 to Troue discloses a treatment enclosure for the continuous in-line irradiation treatment of the surface of a moving coated product. The treatment enclosure includes means for maintaining the surface of a moving coated product under a blanket of inert gas during the irradiation treatment thereof. Troue discusses the importance of the following features regarding inert gas blanketing: That the inert gas flow be laminar; that there be a long entrance tunnel from ambient air which surrounds the enclosure to the source of the inert gas flow and that the gas flow be directed downward toward the surface of the moving coated product.
U.S. Pat. No. 4,448,616 to Francis, Jr., et al. relates to a process for substantially reducing the backmixing or backflow of gases into metal heat treating furnaces by the use of a particular gas jet arrangement and a defined gas flow rate. The gas jet arrangement comprises a pipe with holes which produces a turbulent flow under most conditions of operation. The hole size or width of a slot in the gas distribution conduit is specifically stated not to effect performance of the gas jet in reducing backmixing.
U.S. Pat. No. 4,696,226 to Witmer describes a fluid barrier curtain at an aperture in a wall within a duct, as at the entrance of a furnace. The fluid barrier curtain is used to maintain separation of fluids on opposite sides of the barrier curtain. Witmer discusses the importance of the following features regarding an effective barrier curtain: Having an apparatus which emits a laminar sheet of fluid flow across the aperture zone; the apparatus comprising means for forcing fluid into one side of the aperture zone while removing fluid from the other side of the zone, including the use of thin edge vanes located at the side of the apparatus from which the fluid is removed; and, the relationship between the width of the slot in the fluid curtain emitter and the aperture zone distance across which the fluid enters and exits, e.g. the distance across the aperture zone can be as great as thirty times the width of the slot in the fluid curtain emitter.
Several of the general principals of fluid dynamics which provide background information related to the present invention may be found in Streeter, "Handbook of Fluid Dynamics", McGraw-Hill, New York, 1961 in Section 10, pages 1-33 and in Section 26, pages 1-21.
The design of the apparatus used to prevent an external fluid from entering a process space can vary, as illustrated by the apparatus disclosed in the patents listed above. State of the art technology has permitted the reduction of fluid contaminants within the process space to average concentrations as low as about 100 ppm, with concurrent reductions in total flow of process fluid through openings to the process. The 100 ppm concentration is the normal process condition, with random incursions occurring, during which contaminant concentration can rise as high as ambient concentrations (10.sup.6 ppm).
There are some applications for which a 100 ppm contamination level, and particularly random incursions as high as 10.sup.6 ppm result in product defects or reduced yield. Examples of these applications, not intended to be limiting, follow. Semiconductor manufacturing must be done in a particle-free environment, due to submicron size dimensions in electrical circuitry which can be rendered inoperative by the presence of a particle of dust. Heat-treating, joining and forming of metals requires oxygen-free gaseous environment at elevated temperature, since presence of oxygen (even at concentrations as low as 10 ppm) can cause parts to discolor so that they must be reworked or pickled. In the case of brazing, presence of oxygen may prevent joining from occurring so that the part is ruined. Thick film firing of printed circuits frequently requires several different processing zones in series, with each zone comprising a different atmospheric composition. Molten metal baths such as those used for soldering or galvanizing require protection from oxygen; current technology requires placing an enclosure over the bath and purging it with an inert or reducing gas, or placing an inert liquid atop the molten metal surface. These techniques restrict access to the molten metal surface, cause contamination and substantially increase the operating costs of the process. In addition, there are applications wherein it is desired to protect a window from obscuration by a dirty environment.
The known technology, prior to the present invention, permits incursions of the type previously described due to lack of flow stability. The incursions, due to the resultant high concentration of contaminants, cause substantial damage to the parts or materials being processed, reducing yields and increasing processing costs. Frequently the lack of flow stability is the result of characteristics inherent in the design and operation of the barrier curtain itself.
It is desired to provide a method for reducing fluid and particulate contamination within any given volume of a process space or at the surface of a barrier plane to concentrations at least below 100 ppm while minimizing process fluid flow consumption. In addition, it is desired to provide improved process stability by substantially reducing or eliminating random incursions of contaminants, which in many cases cause more damage to the process or to the surface contacted than the overall average concentration of contaminates. It is also desired to reduce or eliminate the need to use a purge fluid from within the processes space as a method of preventing the entrance of ambient into the process space, since loss or use of purge field from within the process is typically expensive.